An optical multiplexer is a device that combines several input optical signals, each at a different wavelength (frequency), and each entering the device from separate optical fiber inputs, into one output optical signal on one output optical fiber. Similarly, an optical demultiplexer is a device that separates several input optical signals, each at a different wavelength and entering the device via a single optical fiber, into several optical fiber outputs, one for each wavelength. The same device can be used either as an optical multiplexer or as an optical demultiplexer.
In typical wavelength-division multiplexed (WDM) optical networks, a plurality of signals, each at a separate wavelength, are multiplexed by means of one or more optical multiplexers onto a single optical fiber. After being transmitted over the fiber, the signals are separated by means of an optical demultiplexer and directed to individual receivers. In most point-to-point and star WDM networks, multiplexers are distributed among network nodes.
In WDM networks, one or more optical service channels are employed to convey OAM&P (operation, administration, maintenance and provisioning) information between the network nodes and optical amplifier sites. The optical service channels typically use an optical source at a wavelength that is distinct from the wavelengths of the various signals. The bandwidth or data rate of the service channel is typically smaller than the signal bandwidth or data rate.
Generally, one filter is used to add an optical service channel and another is used to drop the optical service channel. Such an optical service channel implementation is disclosed in U.S. Pat. No. 5,113,459 issued May 12, 1992 to Grasso et al., wherein a wavelength substantially different from the wavelength used for the signals is used for optical service channel transmission. Such an implementation is also shown in U.S. Pat. Nos. 5,291,326 (issued Mar. 1, 1994 to Heidemann) and U.S. Pat. No. 5,532,864 (issued Jul. 2, 1996 to Alexander et al.), where a laser outside of the EDFA (Erbium Doped Fiber Amplifier) band is employed. The purported advantage of such a laser is that the service channel survives an amplifier fault.
Numerous additional systems and methods have been proposed. However, these typically add ancillary components (such as filters), escalate the cost of the system, or compromise bandwidth.
For a particular link, the qualities of the transmitter and receiver may be summarised in a single number, called a "link budget". The link budget represents the amount of allowable attenuation of a signal over the link, say 30 dB. If an ancillary component is included in a link, for instance for service channel transmission, additional attenuation may be introduced. If a component reduces signal power by 1 dB, the link budget would then be 29 dB. The 1 dB reduction in link budget is called a "link budget penalty".
It would be desirable to have a method of transmitting information between nodes in a network using a multiplexer and an optical source where no link budget penalty is realized and where a more economical optical source could be employed.